In the post ‘LED 7-Segment Multiplexing‘, I have explained about the concept and benefits of multiplexing. Multiplexing implementation is very similar to driving Led Dot Matrix. I use Timer0 interrupt for switching through each digit. Timer0 or TMR0 is an 8-bit timer which overflows every 256 (0xFF) counts. It’s known that the refresh rate above 50Hz would be enough for human’s eyes to see the display without recognizing the flickering. If I set TMR0 with 1:8 Prescaler, the multiplexing frequency will be
4MHz(internal OSC.)/4(working OSC)/8(prescaler)/256(max counts of TMR0)/6(number of digits) = 81.3 Hz which is good for a display.
Just an example, I have implemented (in Proteus) a 999999-second counter by using 6 Digits LED 7-Segment Multiplexing technique. There are 2 main components in the project, PIC16F627A or PIC16F628 and 6 x LED7-segment display. The schematic shows below. The crystal is 32.768KHz as usual. There is a 10KOhm pull up resistor at RA4 pin as this pin is an open-drain pin as I described in “Open-Drain RA4 pin on PIC Microcontroller“.
The source code in MikroC is listed below: (.hex is also available, please feel free to contact me)
//PIC16F627A
//4MHz Internal OSC
//MUX by the MUC itself with Interrupt
//TMR0 .. check the prescelar+delay in scan routine as they are related
//[email protected]
unsigned short number [10] = {
0x5F, 0x06, 0x9b, 0x8f, 0xC6, 0xCd, 0xDD, 0x07,
0xDf, 0xCf
};
unsigned short digit [6];
unsigned short counter;
unsigned short shift_register;
unsigned short x1;
unsigned short x2;
unsigned short x3;
unsigned short x4;
unsigned short x5;
unsigned short x6;
unsigned short tick;
void interrupt ()
{
if (INTCON.T0IF)
{
//Scan digits with TMR0
INTCON.T0IF = 0;
if (counter == 5)
{
PORTA = number [digit [counter]];
Delay_us (500);
shift_register = 0x01;
PORTB = ~shift_register;
PORTA = 0x00;
counter = 0;
} else
{
PORTA = number [digit [counter]];
Delay_us (500);
shift_register = shift_register << 1;
PORTB = ~shift_register;
PORTA = 0x00;
counter ++;
}
}
if (PIR1.TMR1IF)
{
TMR1H = 0x80;
PIR1.TMR1IF = 0;
tick = 1;
//update current time
x6 ++;
if (x6 > 9)
{
x6 = 0;
x5 ++;
if (x5 > 9)
{
x5 = 0;
x4 ++;
if (x4 > 9)
{
x4 = 0;
x3 ++;
if (x3 > 9)
{
x3 = 0;
x2 ++;
if (x2 > 9)
{
x2 = 0;
x1 ++;
if (x1 > 9)
{
x1 = 0;
}
}
}
}
}
}
}
}
void main ()
{
//Digital I/O for PORTA
CMCON = 0x07;
TRISA = 0x00;
PORTA = 0x00;
TRISB = 0x00;
PORTB = 0x00;
//Internal Clock 4MHz
PCON.OSCF = 1;
counter = 0;
// Enable TMR0
OPTION_REG.T0CS = 0;
// Enable Prescaler
OPTION_REG.PSA = 0;
// PS0,1,2 = 010 = 3
// 3 means 1:8 prescaler
// 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, 1:128, 1:256
OPTION_REG.PS2 = 0;
OPTION_REG.PS1 = 1;
OPTION_REG.PS0 = 0;
INTCON.T0IF = 0;
INTCON.T0IE = 1;
INTCON.GIE = 1;
INTCON.PEIE = 1;
T1CON = 0x0F;
TMR1H = 0x80;
TMR1L = 0x00;
// Enable TMR1 interrupt
PIE1.TMR1IE = 1;
shift_register = 0x01;
x1 = 0;
x2 = 0;
x3 = 0;
x4 = 0;
x5 = 0;
x6 = 0;
while (1)
{
if (tick)
{
tick = 0;
//update digits
digit [0] = x1;
digit [1] = x2;
digit [2] = x3;
digit [3] = x4;
digit [4] = x5;
digit [5] = x6;
}
}
}
Source : 6 Digits LED 7-Segment Multiplexing using PIC16F627A
